It is advisable to equip a respirator with an audio and/or visual alarm that indicates when it is not operating properly. In particular it is necessary to raise an alarm when:
1. The patient becomes disconnected. PA1 2. The pressure of the gas at the supply drops, for instance as a bottle runs out. PA1 3. The respirator itself stops working from equipment failure. PA1 4. The oxygen level in the incoming gas is insufficient.
In large fixed installations these alamrs are electrical or electronic. This is possible when a reliable source of electricity is present, but such electrically powered equipment is not recommended for use in the field, as in an ambulance or helicopter, because a battery must be provided and batteries are traditional sources of failure of portable equipment.
It is therefore advisable to use a pneumatically powered alarm for such portable equipment as if the equipment is operational it disposes of a source of gas under pressure. Unfortunately the only such alarms that are currently available are excessively complex and consume too much of the supply gas just to support the alarm function, even when in standby mode and not emitting an alarm. As a result portable equipment must either use a failure-prone electrical system or dispense with an alarm system altogether.